actually, i'm pretty sure the "gameboy" parts of the GBA would have to be in the GB Micro, since the GBA itself runs on a ARM7 (or 8 i think it is) as main processor, and a Z80 as a secondary co-processors mostly used for sound, and as a main processor for GB/GBC games, which is the same processor that the GBC had, sooooo i doubt that the GB Micro wouldnt have that since the GBA uses it for sound and etc.
i think that the Micro even has the GBC bios and boot in it, just not enabled,
the only difference i'd see is that GBA runs on 3volt and GBC on 5volt, so it would need a power regulator/converter, which is the only thing i think they would have removed =/
as for the GBC palettes, i think it only has like 10 specific palettes (where it checks the game's header) and then 12 basic all purpose palettes (the ones you set with D-PAD + A/B combiations) which i thought the SGB would also have.
it also disappointed me when i found out the SBG2 didnt support GBC games -_________-
so yeah, idk